Specific aims are to assess under what circumstances and with what uncertainty current fecundity status and fecundability can be determined from conventional fertility survey data; the variation of fecundability within and across countries; some of the factors associated with these variations in fecundability; how the use and time of initiation of contraception is influenced by fecundability; and the potential of the measure fo fecundability for assessing births averted by contraception. The data for this analysis will be drawn from the World Fertility Survey, the National Surveys of Family growth, and the Contraceptive Prevalence Surveys, the National Surveys of Family Growth, and the Contraceptive Prevalence Surveys. The methodology will rely heavily on life-table procedures and will exploit the standardized World Fertility Survey data tapes for international comparative purposes. The disciplines involved are primarily demography and statistics. Both investigators have had extensive experience working with WFS data and have conducted a considerable amount of preliminary work on the subject of fecundability and contraceptive efficacy.